The present invention generally relates to fluid supply systems for torque transmitting apparatus of the disc type wherein films of fluid are provided between adjacent discs of a clutch for purposes of transmitting torque and to a method of converting a dry or wet clutch to provide oil shear or hydroviscous operation and torque transmission.
One of the most widely used means of transmitting torque from one element to another in an industrial type machine is a friction disc type clutch or brake device. Such devices operate to transmit torque between the discs thereof by virtue of a pressure applied to the discs and the frictional characteristics of the discs. The total torque transmitted depends on the number of friction faces involved, the mean radius of the friction faces, the pressure on the discs, and the coefficient of friction between the disc faces.
In most applications, there are occurrences of relative motion between the discs while pressure is being applied to them. Examples of such relative motion include clutch or brake slippage during acceleration or deceleration of one machine member relative to another. An example of continuous slip is a tensioning or speed controlling situation where one machine member continually runs at a rotational rate different from that of a second member while the clutch transmits a regulated amount of torque. In all of these applications, heat is generated at the friction faces of the discs. This heat must be kept within reasonable limits, or there will be a rapid deterioration of the friction faces or other structural clutch damage.
Frequently, clutches or brakes are operated in an oil spray or within a housing containing a fluid which usually comprises an oil so that the discs dip into the fluid as they rotate. Such uses are termed "wet" clutch or brake applications. This is an effective means of keeping disc temperatures down for installations where the clutch or brake is energized to accelerate or decelerate a machine member. In wet applications, a film of fluid is present on the faces of the discs as the clutch or brake closes, and the heat energy is primarily generated in this fluid film. The heated fluid is then squeezed out by clutch pressure, aided by centrifugal forces of the rotating discs, thus carrying heat away from the discs.
In wet applications, the film cooling principle is limited by the amount of fluid that can be contained as a film on the discs when they close or the amount of fluid that can be obtained between the discs when they close. To further increase the heat dissipating capability of such devices, particularly in tensioning or speed controlling applications, it is practical to provide a means of introducing fresh, cool fluid between the discs as they rotate. In such cases, a film of fluid is maintained between the discs.
The prior art discloses that the torque transmitted by such a film of fluid between friction discs is varied or controlled by the axial closing pressure on the discs, which in turn varies the thickness of the film between the discs. In essence, torque is transmitted by the shearing of this fluid film, and torque is increased by increasing the axial closing pressure applied to the discs, and consequently decreasing the thickness of the fluid film between the discs. Since the film heated by the shearing forces is replaced by fresh, cool fluid introduced between the discs, the heat generated is carried away with the displaced fluid. This type of operation is termed "oil shear operation" when the fluid film between discs is reinforced by cool fluid, while the clutch or brake opens and closes and is used to accelerate or decelerate a machine member. In more severe applications where relative slip is continuous, such as tensioning or speed control applications, the process is termed "hydroviscous operation." These two operations are fundamentally similar, except that continuous slip hydroviscous operation requires a higher level of fluid flow; however, a system designed for hydroviscous operation can always be used for oil shear performance.
The maintenance of an adequate supply of fluid to the discs for purposes of assuring hydroviscous operation and dissipation of the heat energy generated during the shearing of the films of fluid between the discs is a primary concern in such hydroviscous torque transmitting units. To that end, the prior art discloses the submersion of the torque transmitting unit within a housing-defined reservoir of oil and the use of pumps to deliver oil to the discs at a positive pressure. In addition, external oil cooling apparatus such as heat exchangers may also be employed in particularly severe applications such as when high cyclic rates, high load inertias, or constant slip are involved.
The use of pumps external to the housing of the unit to deliver oil to the discs involves additional pump drive and control apparatus and frequently entails extensive drilling of a shaft of the unit. Similarly, pumps mounted within the housing unit at a location remote from the torque transmitting unit itself are not entirely satisfactory, since they require additional pump mounting structure and oil lines for delivery of the oil to the discs from the remote location. In resolution of such problems, integrally mounted centrifugal pumps secured directly to a rotating portion of the clutch unit are disclosed in applicant's U.S. Pat. No. 3,912,060, dated Oct. 14, 1975, which is incorporated herein by reference. The integrally mounted pump comprises an annular shroud which encircles the shaft and serves as a pump housing defining an intake port about the shaft. The annular shroud includes radial vanes for pumping oil drawn from the surrounding reservoir through fluid ducts which communicate with the inner portions of the discs.
Other prior art arrangements have used a cavity surrounding the bore or shaft of the clutch, with the cavity connecting to axially extending fluid passages. This cavity arrangement is similar in operation to the previously described pump shroud arrangement, except that the pump shroud design draws fluid from an ambient reservoir, and the cavity arrangement is designed to have cooling fluid introduced through a supply pipe from a separate pumping system. In either case, this requirement of supplying the fluid to the cavity or shroud in the space between the shaft and the central inside opening of the cavity or shroud precludes the extension of the axially extending passages to a location in close proximity within the shaft or bore of the clutch. Space must be left around the shaft to provide an inlet for the supply of fluid.
Assuming given ambient conditions, fluid, discharge head or flow resistance, and pump impeller rotational rate, the characteristics of a centrifugal pump that determine its effective output are: the diameter of the impeller, the internal pump friction, and the flow characteristics including its ability to minimize internal cavitation. The diameter of the impeller establishes a centrifugal pump pressure potential or maximum pressure the pump can develop, and the physical design characteristics determine a centrifugal pump potential or maximum effective pump output or flow rate capability. Herein, an integral centrifugal pump or a passageway pump system are each considered to have a centrifugal pump pressure potential in accordance with their diameters or radius dimensions. Similarly, such integral or passageway pumps are also each deemed to have a centrifugal pump potential or pump flow rate capability in accordance with the overall physical design characteristics of the particular pump.
The integral centrifugal pump is generally sized to maximize the shroud or pump housing diameter in order to maximize the pump capability. Accordingly, the shroud or pump housing of the integrally mounted pump is selected to correspond in size or diameter with the diameter of the rotating portion of the clutch to which it is to be secured. In practice, the maximum pump diameter is that which corresponds with the diameter of the innermost regions of the discs, and the torque transmitting units or clutches of concern herein have a centrifugal pump or pumping potential which is determined by the radial distance from the axis of rotation or shaft bore to the innermost regions of the discs. As indicated, the optimum pump diameter is equal to the diameter of the innermost region of the discs and axially extending fluid ducts are used to deliver the fluid to the discs. In many instances, the pump diameter of an integrally mounted pump is limited by stationary clutch structure located between the shaft bore and the discs, and the fluid ducts will include radially extending portions to reach the discs.